A sample of GRB radio afterglows inconsistent with the standard jet model

We present a sample of 15 gamma-ray burst (GRB) afterglow light curves at radio frequencies, and compare them to the X-ray and/or optical properties of the afterglows and to the predictions of the standard jet/fireball model. Our sample has been chosen so that each afterglow exhibits a jet break at some frequency, usually X-ray. We examine the late-time decline of each burst in the radio and in the X-ray, and attempt to fit an analytical model based on the conventional GRB afterglow equations to each data set. We show that the GRBs in our sample are mostly incompatible with the light curve behavior predicted by conventional GRB afterglow theory. In particular, we observe a lack of visible jet breaks in the radio light curve, even when one is seen in the X-ray. No radio afterglow in this sample, at any time, shows the expected post-break decline of $\sim t^{-2}$, although a few remain consistent with the standard model if such a decline began soon after the observations. The observed decline in the radio is often described at least as well by a single power law as by the standard model, in some cases being consistent with the expected pre-break decline (assuming $\nu_{\mathrm{radio}} > \nu_m$) until late times. Notably, signs of a jet break are visible in the millimeter-wave afterglow of GRB 161219B and GRB 111215A, perhaps suggesting that only lower radio-frequency afterglows behave anomalously. Nonetheless, the observed behavior conflicts with our current theoretical understanding of radio afterglows.